High-aluminum-containing ferritic stainless steel having improved high-temperature oxidation resistance

ABSTRACT

A ferric stainless steel consists by weight of less than 0.03% carbon, less than 1% silicon, less than 1% manganese, less than 0.04% phosphorus, less than 0.03% sulphur, from 15% to 25% chromium, less than 0.03% nitrogen, from 3% to 6% aluminum, from 0.1% to 4% molybdenum. A small amount of rare-earth elements and yttrium of from 0.01% to 0.15% are added in the steel.

BACKGROUND OF THE INVENTION

The present invention relates to a high-aluminum-containing ferriticstainless steel which is used at high-temperature such as a catalyst inan emission control system for motor vehicles, a heating device, andothers.

The high-aluminum ferritic stainless steel has a high oxidationresistance at high temperature so that it is extensively used formaterials in heating devices such as a stove pipe, and electric heaterelements.

Ceramics have heretofore been used as a catalyst carrier of a catalyticconverter provided in an emission control system for motor vehicles, andferritic stainless steel is now beginning to be used instead. One of thedefects of the ceramic is that the ceramic is vulnerable to thermalshock. Another is that the ceramic has a high heat capacity.Consequently, when the ceramic is used for a catalyst carrier, a longtime is required to be heated to a catalyst reacting temperature. Ametallic carrier made of high-aluminum ferritic stainless steel has notsuch defects as the ceramic.

The stainless steel is formed into a sheet of foil having a thickness ofabout 50 μm to be used as a catalyst carrier for catalytic converters.Since abnormal oxidation is liable to occur on the foil and theconverter is used in an atmosphere of exhaust gases which is severeoxidization conditions, the foil must have extremely resistivecharacteristics against high-temperature oxidation. To meet therequirements, a high-aluminum-containing ferritic stainless steelincluding 20Cr-5Al as a base, rare-earth elements and yttrium is used.However, such a stainless steel does not have sufficienthigh-temperature oxidation resistance, so that an extended use of thesteel causes abnormal oxidation.

In addition, from the point of global warming and public nuisance,stricter limits have been imposed on emission control. To comply withthe standards, it is necessary to quickly heat the catalyst to thecatalyst reacting temperature after the start of the engine. To thisend, the temperature of exhaust gas is elevated, or the converter ispositioned directly under an exhaust manifold. On the other hand, thepower of the engine tends to be increased in recent years, which causesalso increase of the temperature of the exhaust. These conditionsenhance oxidation and corrosion of the catalyst carrier. Therefore, itis imperative to provide a high-aluminum-containing ferritic stainlesssteel having improved high-temperature oxidation resistance.

In order to improve the resistance of the high-aluminum-containingferritic stainless steel against high-temperature oxidation, contents ofchrominum, aluminum, rare earth elements and yttrium are increased,which is disclosed in Japanese Patent Application Laid-Open 63-45351.However, slabs and hot-rolled plates made of such a stainless steel arepoor in toughness and hence, in productivity. More particularly,increase in chromium and aluminum contents causes not only rise in thecost of the component material, but also decreases productivity due topoor toughness. As a consequence the yield of the stainless steeldecreases, thereby extremely increasing the manufacturing cost, or theproduction becomes altogether impossible. Alternatively, the rare-earthelements and yttrium are added to promote high-temperature oxidationresistance. However, excessive quantity of these elements ratherdecreases the high-temperature oxidation resistance, and causesdeterioration in toughness.

In addition, the ferritic stainless steel foil of the catalyticconverter is subjected to heat cycles where heating and cooling arerepeated, thereby causing deformation of the foil. Thus, the catalystcarrier must have high heat resistance.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a ferritic stainlesssteel where the high-temperature oxidation resistance is improvedwithout increasing contents of components, thereby improving productionof the steel.

According to the present invention, the material of a high-aluminumferritic stainless steel having high-temperature oxidation resistancecontains by weight under 0.03% carbon, under 1% silicon, under 1%manganese, under 0.04% phosphorus, under 0.003% sulphur, from 15% to 25%chromium, under 0.03% nitrogen, from 3% to 6% aluminum, from 0.1% to 4%molybdenum, and from 0.01% to 0.15% in total of one or more rare-earthelements and yttrium.

By weight of 0.05% to 1% in total of one or more of niobium, vanadiumand titanium may be arbitarily included in the material.

Namely, when molybdenum and small quantities of rare earth elements andyttrium are included in the high-aluminum containing ferritic stainlesssteel, high-temperature oxidation resistance and heat resistance areimproved so as to be applicable to use as a catalyst carrier in anemission control system for motor vehicles, heating devices and electricheater elements.

The other objects and features of this invention will become understoodfrom the following description with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

The figure is the graph showing a relationship between the quantity ofmolybdenum and the time elapsed before the occurrence of abnormaloxidation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Content of eachcomponent and reason for numerical limitation of the content aredescribed hereinafter. Carbon C

Excessive content of carbon in high-aluminum-containing ferriticstainless steel induces abnormal oxidation and decreases toughness ofthe slabs and hot foils. The decrease of the toughness causes reductionof productivity of the steel. Therefore, the maximum content of thecarbon is limited to 0.03% by weight in the present invention.

Silicon Si

Silicon increases the hardness of the ferritic stainless steel and hencedecreases the toughness thereof so that the content thereof is under 1%by weight

Manganese Mn

Manganese, although effective in enhancing hot workability, decreasesthe high-temperature oxidation resistance. Thus the content of themanganeses is limited to under 1% by weight.

Phosphorus P

Since phosphorus affects the high-temperature oxidation resistance, itis preferable to limit the content thereof as small as possible. Inaddition, phophrorus also reduces toughness of the hot-rolled plate.Thus, the content of phosphorus is set under 0.04% by weight.

Sulphur S

Sulphur combines with rare-earth elements and yttrium to form anonmetallic inclusion which affects the surface quality of stainlesssteel. Sulphur may further decrease the effective quantities of the rareearth elements and yttrium which are effective in improving oxidationresistance. The significance of theses defects intensifies if over0.003% by weight of sulphur is included. Hence under 0.003% by weight,preferably under 0.002% by weight of sulphur is included.

Chromium Cr

Chromium is a basic element for improving high-temperature oxidationresistance. Content in excess of 15% by weight of chronium is necessaryfor effectuating the improvement. However, if chromium in excess of 25%by weight is included, toughness of slab and hot foil deteriorates,thereby reducing productivity. Hence the chromium content is between 15%to 25% by weight.

Nitrogen N

Nitrogen, when reacted with aluminum, forms aluminum nitride (AlN) whichinduces abnormal oxidation. As the content of nitrogen is increased,toughness of the stainless steel deteriorates. Hence the content ofnitrogen is under 0.03% by weight.

Aluminum Al

Aluminum, as well as chromium, is a significant element in maintainingthe high-temperature oxidation resistance of the stainless steel. Moreparticularly, an aluminum oxide film (Al₂ O₃) is formed on the surfaceof the stainless steel by adding aluminum. The aluminum oxide filmpromotes the resistance. In order to restrain abnormal oxidation, whichis liable to occur on foil having a thickness under 100 μm, aluminumcontents over 3% by weight should be included so as to form sufficientaluminum oxide film on the surface. However, if the content of aluminumexceeds 6% by weight, the toughness of slab and hot foil deteriorates.Thus, the aluminum content is from 3% to 6% by weight.

Molybdenum Mo

Since molybdenum causes a highly volatile oxide, it has been thought asan element which aggravates the high-temperature resistance. However,the inventors of the present invention have found to the contrary thatmolybdenum improves the high-temperature resistance, as well as heatresistance of the steel. The effect becomes apparent when the molybdenumcontent is over 0.1% by weight as shown in the graph of the figure.However, if the content exceeds 4% by weight, toughness of the stainlesssteel decreases, thereby affecting productivity. Hence the molybdenumcontent is between 0.1% and 4% by weight.

Rare Earth Elements and Yttrium

Rare earth elements and yttrium are important elements in improvinghigh-temperature oxidation resistance in iron-chromium-aluminumstainless steels. Rare earth elements such as lanthanum and cerium, andyttrium are effective in stabilizing aluminum oxide (Al₂ O₃) film formedon the surface of the stainless steel, and hence in restraining abnormaloxidation which is liable to occur on the stainless steel foil. Inaddition, the oxide film on the steel base is stabilized by the rareearth elements and yttrium. These effects become significant if over0.01% by weight of rare earth elements and yttrium are included intotal. If the contents exceed 0.15% by weight, hot workability andtoughness decrease. Moreover, nonmetallic inclusions which induce theabnormal oxidation is easily formed. As a result, the high-temperatureoxidation resistance is deteriorated contrary to the intention.Therefore, one or two of the rare earth elements and yttrium areincluded in the range of 0.01% to 0.15% by weight in total.

Niobium, Vanadium and Titanium Nb, V, Ti

Adequate quantities of niobium, vanadium and titanium are arbitrarilyincluded in the stainless steel to be bonded with carbon and nitrogentherein so that the toughness significantly improved. Niobium, vanadiumand titanium are also effective in improving heat resistance of themetallic carrier of a catalyst converter which tends to be deformedthrough repeated heating and cooling in heat cycles. In order to preventthe deformation, it is necessary to include one or two of niobium,vanadium and titanium in content over 0.05% by weight in total. However,if the content exceeds 1% by weight, the hardness of the stainless steelbecomes too high. Thus, 0.05% to 1% of niobium, vanadium and titanium ispreferably included.

Examples of the present invention are described hereinafter.

High-temperature oxidation resistance of the high-aluminum-containingferritic stainless steel is imparted by an aluminum oxide film layerformed on the surface thereof. In order to stabilize the aluminum oxidefilm, it is effective to increase the content of chromium. However, inan ordinary iron-chormium-aluminum stainless steel, the oxide film isnot sufficiently adhered, so that the film may flake off during acooling process of the steel, causing abnormal oxidation.

Table 1 shows results of oxidation tests performed on various ferriticstainless steel to examine influence of the components on time at whichabnormal oxidation occurs. Each of the ferritic stainless steel testpieces has a thickness of 50 μm. The test pieces were inserted in aheating furnance disposed in the atmosphere and maintained at atemperature of 1150° C. The test pieces were taken out of the furnancesometimes and observed by the naked eye for inspecting abnormal oxide inthe form of projection other than normally existing thin and uniformoxide film. The abnormal oxidation occurrence time shown in the tableindicates a length of time until the abnormal oxide occurs. As apparentfrom the Table 1, the stainless steel containing rare-earth elements(REM) or yttrium has far longer abnormal oxidation occurrence time than20Cr-5Al steel (test piece A), thus showing improved high-temperatureoxidation resistance.

                  TABLE 1                                                         ______________________________________                                        TEST PIECE        A        B        C                                         ______________________________________                                        COMPONENT AND CON-                                                            TENT (WEIGHT PERCENT)                                                         C                 0.014    0.023    0.012                                     Si                0.34     0.32     0.32                                      Mn                0.32     0.35     0.31                                      P                 0.025    0.023    0.025                                     S                 0.0022   0.0019   0.0022                                    Cr                20.05    20.02    20.09                                     N                 0.014    0.014    0.012                                     Al                4.87     5.01     5.02                                      REM               --       0.06     --                                        Y                 --       --       0.05                                      ABNORMAL OXIDATION                                                                              45       210      250                                       OCCURRENCE TIME                                                               ______________________________________                                    

Heretofore steel having an oxidation resistance withstanding 100 hoursat 1150° C. was used for a catalyst converter carrier. However, theenvironment in which the present ferritic stainless steel is used hasbecome much more severe than before as hereinbefore described.Therefore, it is presumed that the catalyst carrier has an abnormaloxidation occurrence time more than 300 hours. Hence the oxidationresistance of the stainless steels shown in Table 1 is not sufficient.

Accordingly, as shown in Table 2, the quantities of aluminum, yttriumand rare-earth elements which are effective in improvinghigh-temperature oxidation resistance were increased. The steels weremelted in a 30 kg vacuum melting furnance and cast into a steel ingot.However, when the ingot was hot forged, the steel was cracked. Namely,the addition of excessive quantities of aluminum, yttrium and rare-earthelements proved to be an inadequate solution from the point ofproductivity.

                  TABLE 2                                                         ______________________________________                                        TEST PIECE     P         Q         R                                          ______________________________________                                        COMPONENT AND                                                                 CONTENT                                                                       (WEIGHT PERCENT)                                                              C              0.012     0.011     0.014                                      Si             0.33      0.30      0.30                                       Mn             0.37      0.39      0.39                                       P              0.025     0.025     0.025                                      S              0.0020    0.0019    0.0020                                     Cr             20.01     20.04     20.09                                      N              0.012     0.011     0.014                                      Al             5.08      6.25      5.07                                       REM            0.15      0.09      --                                         Y              0.03      --        0.09                                       RESULTS OF     CRACKS    CRACKS    CRACKS                                     FORGING        FOUND     FOUND     FOUND                                      ______________________________________                                    

The inventors of the present invention have conducted various studiesand experiments to obtain a steel having an improved high-temperatureoxidation resistance without increasing the contents of aluminum,yttrium and rare earth elements. Namely, influence of molybdenum onhigh-temperature oxidation resistance was investigated at a temperatureof 1150° C. using ferritic stainless steel foils having a thickness of50 μm. The stainless steels were also wrought into honeycomb carriersfor a catalytic converter having a diameter of 50 mm, length of 100mmand a height of 1.5 mm. One-hundred cooling and heating cycles in whichthe test pieces were subjected to at temperatures of 900° C. and 200° C.for thirty minutes, respectively, were performed to examine thedeformations thereof.

                                      TABLE 3                                     __________________________________________________________________________    TEST PIECE       D    E       F       G       H       I                       __________________________________________________________________________    COMPONENT AND CONTENT                                                         (WEIGHT PERCENT)                                                              C                0.026                                                                              0.021   0.023   0.014   0.015   0.013                   Si               0.31 0.31    0.32    0.31    0.34    0.30                    Mn               0.27 0.24    0.25    0.29    0.25    0.30                    P                0.024                                                                              0.024   0.024   0.025   0.024   0.025                   S                0.0019                                                                             0.0018  0.0021  0.0023  0.0021  0.0020                  Cr               20.04                                                                              20.08   20.03   20.09   20.10   20.07                   N                0.013                                                                              0.011   0.013   0.012   0.010   0.012                   Al               5.09 5.10    5.11    5.10    5.13    5.09                    REM              0.09 0.09    0.09    0.08    0.09    0.08                    Mo               --   0.19    0.51    1.13    2.35    3.87                    ABNORMAL OXIDATION                                                                             240  310     330     500     610     740                     OCCURRENCE TIME                                                               DEFORMATION      FOUND                                                                              NOT FOUND                                                                             NOT FOUND                                                                             NOT FOUND                                                                             NOT FOUND                                                                             NOT                     __________________________________________________________________________                                                          FOUND               

As shown in Table 3, the abnormal oxidation occurrence time in each ofthe test pieces which includes molybdenum was extremely extended, thatis, the high-temperature oxidation resistance was remarkably improved.The results show that the molybdenum acts to enhance the oxidationresistance characteristic of the aluminum oxide (Al₂ O₃) formed on theiron-chromium-aluminum stainless steel including rare-earth elements andyttrium. The reason of the improvement in the high-temperature oxideresistance is not quite clear but is assumed that molybdenum dissolvesin the aluminum oxide film, thereby covering defects of the oxide film,preventing the oxygen from entering. Additionally, deformation of thesteel did not occur through the cooling and heating cycles so that itcan be said that the heat resistance is also improved.

Thus, the high-aluminum-containing ferritic stainless steel of thepresent invention including molybdenum has better resistance againsthigh-temperature oxidation and has a high-temperature strength comparedto a conventional iron-chromium-aluminum stainless steel with or withoutrare earth elements and yttrium.

The examples of the ferritic stainless according to the presentinvention are described hereinafter.

Each of components of the ferritic stainless steel listed in Table 4 wasvacuum-melted, forged, cut and hot-rolled, and thereafter repeatedlyannealed and cold-rolled to form a piece of foil having a thickness of50 μm. The test piece was heated for the oxidation test at 1150° C.until oxidation was observed. The stainless steel foil was furtherwrought into a honeycomb carrier for a catalytic converter. Five-hundredcooling and heating cycles each of which comprising keeping the testpiece in an exhaust gas atmosphere at a temperature of 200° C. for fiveminutes and thereafter at a temperature of 900° C. for thirty minutes.The abnormal oxidation occurrence time of each specimen and thedeformation of the honeycomb made from the specimen were examined asshown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________                                                     Abnormal                                                                      Oxidation                    Specimen                                                                            Component and Content (Weight Percent)     Occurrence                   No.   C  Si Mn P  S   Cr Al N  La Ce Y  Mo Remainder                                                                           Time (Hours)                                                                         Deformation           __________________________________________________________________________    Present                                                                       Invention                                                                      1    0.014                                                                            0.35                                                                             0.24                                                                             0.025                                                                            0.0021                                                                            20.09                                                                            5.12                                                                             0.012                                                                            0.03                                                                             0.02                                                                             -- 0.51     320    Not Found              2    0.017                                                                            0.31                                                                             0.22                                                                             0.024                                                                            0.0018                                                                            20.15                                                                            5.09                                                                             0.011                                                                            0.05                                                                             -- -- 1.08     490    Not Found              3    0.019                                                                            0.32                                                                             0.24                                                                             0.024                                                                            0.0019                                                                            20.04                                                                            5.11                                                                             0.010                                                                            -- -- 0.07                                                                             2.09     590    Not Found              4    0.012                                                                            0.33                                                                             0.22                                                                             0.025                                                                            0.0020                                                                            20.06                                                                            4.95                                                                             0.017                                                                            0.02                                                                             0.01                                                                             0.02                                                                             3.12     700    Not Found              5    0.011                                                                            0.30                                                                             0.33                                                                             0.025                                                                            0.0015                                                                            16.03                                                                            5.73                                                                             0.013                                                                            0.06                                                                             -- -- 2.01     420    Not Found              6    0.014                                                                            0.34                                                                             0.26                                                                             0.025                                                                            0.0022                                                                            23.05                                                                            3.26                                                                             0.013                                                                            0.03                                                                             0.02                                                                             0.02                                                                             2.14     380    Not Found              7    0.013                                                                            0.33                                                                             0.53                                                                             0.025                                                                            0.0020                                                                            20.27                                                                            5.13                                                                             0.014                                                                            0.03                                                                             -- 0.02                                                                             2.12                                                                             Nb:                                                                              0.29                                                                             410    Not Found                                                        V: 0.11                             8    0.015                                                                            0.33                                                                             0.27                                                                             0.025                                                                            0.0022                                                                            18.33                                                                            5.04                                                                             0.008                                                                            0.08                                                                             -- -- 1.97                                                                             Ti:                                                                              0.22                                                                             420    Not Found              9    0.011                                                                            0.31                                                                             0.26                                                                             0.025                                                                            0.0021                                                                            20.13                                                                            5.05                                                                             0.012                                                                            0.05                                                                             -- -- 2.10                                                                             V: 0.07                                                                             320    Not Found             10    0.015                                                                            0.35                                                                             0.22                                                                             0.025                                                                            0.0019                                                                            18.15                                                                            4.11                                                                             0.015                                                                            -- 0.06                                                                             -- 3.94                                                                             Ti:                                                                              0.31                                                                             320    Not Found             Compara-                                                                      tive                                                                          Example                                                                       11    0.014                                                                            0.34                                                                             0.24                                                                             0.025                                                                            0.0021                                                                            20.12                                                                            5.02                                                                             0.012                                                                            0.03                                                                             0.03                                                                             -- --       240    Found                 12    0.017                                                                            0.31                                                                             0.31                                                                             0.024                                                                            0.0021                                                                            20.22                                                                            5.19                                                                             0.014                                                                            0.05                                                                             -- -- --       250    Found                 13    0.019                                                                            0.30                                                                             0.33                                                                             0.024                                                                            0.0023                                                                            20.24                                                                            5.01                                                                             0.016                                                                            -- -- 0.05                                                                             --       240    Found                 14    0.012                                                                            0.34                                                                             0.32                                                                             0.025                                                                            0.0020                                                                            20.11                                                                            4.99                                                                             0.017                                                                            0.03                                                                             0.01                                                                             0.02                                                                             --       190    Found                 15    0.011                                                                            0.30                                                                             0.31                                                                             0.025                                                                            0.0015                                                                            16.11                                                                            5.68                                                                             0.013                                                                            0.06                                                                             -- -- --       170    Found                 16    0.014                                                                            0.33                                                                             0.31                                                                             0.025                                                                            0.0019                                                                            22.98                                                                            3.35                                                                             0.012                                                                            0.03                                                                             0.02                                                                             0.02                                                                             --       190    Found                 17    0.013                                                                            0.32                                                                             0.33                                                                             0.025                                                                            0.0023                                                                            20.18                                                                            5.03                                                                             0.021                                                                            0.03                                                                             -- 0.02                                                                             -- Nb:                                                                              0.25                                                                             120    Some Found                                                       V: 0.13                            18    0.016                                                                            0.32                                                                             0.27                                                                             0.025                                                                            0.0021                                                                            18.19                                                                            5.14                                                                             0.011                                                                            0.07                                                                             -- -- -- Ti:                                                                              0.19                                                                             130    Some Found            19    0.013                                                                            0.32                                                                             0.36                                                                             0.025                                                                            0.0021                                                                            20.93                                                                            5.09                                                                             0.010                                                                            0.05                                                                             -- -- -- V: 0.09                                                                             110    Some Found            20    0.014                                                                            0.33                                                                             0.22                                                                             0.025                                                                            0.0023                                                                            18.23                                                                            4.13                                                                             0.010                                                                            -- 0.06                                                                             -- -- Ti:                                                                              0.29                                                                              90    Some                  __________________________________________________________________________                                                            Found             

As is understood from the Table 4, each of the specimens according tothe present invention has excellent high-temperature oxidationresistance. Namely, oxidation did not appear for over 300 hours in theexamples of the present invention, whereas the comparative stainlesssteels without molybdenum assumed abnormal oxidation within 300 hours.Thus, the stainless steel of the present invention satisfies the desiredcharacteristics for a catalyst carrier of an automotive catalyticconverter. Furthermore, no deformation occurred in the test piece of thepresent invention. To the contrary, deformation was observed in thecomparative example, that is, in specimens Nos. 11 to 16. Each of thespecimens Nos. 17 to 20 having niobium, vandium and titanium has aslight effect to prevent the deformation, but is not sufficient.

The components of the stainless steels shown in Table 5 were melted in30kg vacuum melting furnance, forged and annealed. In accordance withthe testing method defined in JIS (Japanese Industrial Standard) G057,test pieces, each having a length of 50 mm at a parallel portion and adiameter of 10 mm were cut out from the ferritic stainless steel. Thetension test for examining 0.2% proof stress and tensile strength wasconducted at 800° C.

                                      TABLE 5                                     __________________________________________________________________________                     PRESENT INVENTION                                                                          COMPARATIVE EXAMPLE                             SPECIMEN No.     21    22     23     24                                       __________________________________________________________________________    COMPONENT AND CONTENT                                                         (WEIGHT PERCENT)                                                              C                0.015 0.013  0.013  0.014                                    Si               0.31  0.31   0.33   0.32                                     Mn               0.23  0.25   0.21   0.18                                     P                0.025 0.024  0.025  0.024                                    S                0.0021                                                                              0.0019 0.0019 0.0022                                   Cr               20.06 20.05  20.08  20.03                                    N                0.015 0.014  0.015  0.013                                    Al               5.11  5.08   5.10   5.12                                     La               0.10  --     --     0.09                                     Y                --    0.09   --     --                                       Mo               2.02  2.12   --     --                                       0.2% PROOF STRESS                                                                              6.8   5.1    4.2    4.4                                      (kg/mm.sup.2)                                                                 TENSILE STRENGTH 9.9   7.2    6.0    6.4                                      (kg/mm.sup.2)                                                                 __________________________________________________________________________

As shown in Table 5, test pieces of specimens Nos. 21 and 22 of thepresent invention had both higher proof stress and tensile strength thanthose of comparative examples, specimens Nos. 23 and 24, thus, showingimproved strength at high temperature.

The test results show that the stainless steel in accordance with thepresent invention has excellent high-temperature oxidation resistance sothat the abnormal oxidation is unlikely to occur. Moreover, thestainless steel has excellent durability to cooling and heating testingcycles and has a sufficient high-temperature strength.

From the foregoing, it will be understood that the present inventionprovides a high-aluminum-containing ferritic stainless steel which hasimproved high-temperature oxidation resistance and high-temperaturestrength without increasing the amounts of chromium, aluminum and rareearth elements. The stainless steel of the present invention is henceextensively suitable for high-temperature applications such as acatalyst carrier of a catalytic converter provided as an emissioncontrol equipment, heating device and electric heating element.

While the presently preferred embodiments of the present invention havebeen shown and described, it is to be understood that these disclosuresare for the purpose of illustration and that various changes andmodifications may be made without departing from the scope of theinvention as set forth in the appended claims.

What is claimed is:
 1. A ferritic steel consisting by weight of lessthan 0.03% carbon, less than 1% silicon, less than 1% manganese, lessthan 0.04% phosphorus, less than 0.003% sulphur, from 15% to 25%chromium, less than 0.03% nitrogen, from 3% to 6% aluminum, from 0.1% to4% molybdenum, from 0.01% to 0.15% at least one of rare-earth elementsand yttrium, and from 0.05% to 1% at least one of niobium, vanadium andtitanium and the balance iron except for inevitable impurities.
 2. Thealloy claimed in claim 1 wherein said molybdenum is present in aproportion of 1 to 4 weight per cent.
 3. A substrate of a catalyticconverter comprising a ferritic stainless steel consisting by weight ofless than 0.03% carbon, less than 1% silicon, less than 1% manganese,less than 0.04% phosphorus, less than 0.003% sulphur, 15-25% chromium,less than 0.03% nitrogen, 3-6% aluminum, 0.1-4% molybdenum, 0.01-0.15%in total at least one of rare earth elements and yttrium, 0.05-1% intotal at least one of niobium, vanadium and titanium, and the balancebeing iron except inevitable impurities.
 4. The catalytic converterclaimed in claim 3 wherein said alloy contains molybdenum in aproportion of 1 to 4 weight per cent.